1. Field of the Invention
The present invention relates to an inner bobbin case holder (hereinafter, often referred to merely as "bobbin case holder") of a fully rotating hook provided in a lockstitch sewing machine or the like, and particularly to the configuration of a track projection thereof.
2. Description of the Related Art
In a bobbin case holder provided in a lockstitch sewing machine or the like, a track projection is formed on an outer surface of the holder, and the track projection is fitted into a track groove formed on an inner surface of an outer loop taker. Only the outer loop taker is rotated at a high speed, while rotation of the bobbin case holder in the downstream direction of the rotation of the outer loop taker is blocked by a rotation restraining member fixed to the body of the sewing machine. Particularly, an outer loop taker for an industrial sewing machine is rotated at a high speed of about 6,000 to 10,000 rpm. Therefore, the sliding contact between the track projection and the track groove generates a large friction force. This necessitates a large torque for rotating the outer loop taker. When the friction between the track projection and the track groove is large, the rotation restraining member blocking the rotation of the bobbin case holder is pressed with a large force by an inner surface of the bobbin case holder into which the rotation restraining member is fitted, thereby impeding smooth loop-spreading of a needle thread. This causes the tension of the needle thread to be unwillingly changed, producing problems in that sewing quality is impaired and that the track projection and the track groove are worn so that durability thereof is lowered. A well known countermeasure to solve these problems is to utilize a configuration in which a lubricant is supplied to the interface between the track projection of the bobbin case holder and the track groove of the outer loop taker. However, this countermeasure has a drawback that needle and bobbin threads and cloth may be stained by the lubricant.
Conventionally therefore, the track projection of the bobbin case holder is coated by a material such as a fluororesin which has a low friction coefficient, or shaped using only a heat resistant and wear resistant resin. As disclosed in Japanese patent publication (Kokai) No. SHO60-64,777, a heat resistant and wear resistant resin may be detachably attached to the track projection of the bobbin case holder.
In the prior art, when fluororesin or the like is to be coated on the bobbin case holder, the resin is sprayed on the holder and then baked. Since the film thickness is generally as thin as about 10 to 20 .mu.m, the durability of the film is low, thereby producing a problem in that the resin film is easily peeled off. When a bobbin case holder is shaped by using only a heat resistant and wear resistant resin, the portion over which a thread passes is worn to a larger degree compared with other portions of the holder so that a so-called thread handing area is formed, thereby producing a problem in that the bobbin case holder is inferior in durability to a holder made of steel. Furthermore, in the ease that a synthetic resin is detachably fitted, the synthetic resin is liable to slip off during operations or causes a trouble such as that a thread is caught in a gap between the synthetic resin and the track projection portion.
It is an object of the invention to provide a bobbin case holder of a fully rotating hook wherein friction between sliding portions of the bobbin case holder and an outer loop taker is reduced as much as possible so that the outer loop taker can rotate smoothly and stably at a high speed.
In a rotating hook for a sewing machine, the track projection of the bobbin case holder is fitted into the track groove of the outer loop taker, and the outer loop taker rotates at a high speed, whereby the sewing operation is performed. It is intended to reduce the friction coefficient between the track projection and the track groove to reduce the torque necessary for rotating the outer loop taker, so that sewing quality can be improved.
It is another object of the invention to provide a bobbin case holder of a sewing machine wherein the friction coefficient between the track projection of the bobbin case holder and the track groove of an outer loop taker is reduced so that the outer loop taker can rotate with a small torque.